I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

I assume your talking about stars? If you are, then its fusion that
keeps that from happening. When the elements in the star (hydrogen,
helium etc) undergo fusion, the sum of their masses before fusion does
not equal the total mass after fusion. This loss of mass is translated
into energy using Einstien's famous equation e = mc^2. This energy
keeps the star hot. Eventually as they burn off all their mass they
cool down into red gians and white dwarfs, but that takes billions of
years. Hope this helps.

curious wrote:

Quote:

Please forgive my ignorance, but:

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

this doesn't happen in real life because the energy acquired by
colliding gas molecules is insufficient to drive the electrons to
higher energy states. Moreover how is that the electrons falling back
to lower states drive the gas to a solid state?

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

That is true. But what is also true is that the reverse events are
possible as well. An electron can get excited by absorbing a photon, as
well as de-excited through a collision. The reverse of any microscopic
event is possible due to the reversibility of the equations of motion
(or, if you know about quantum mechanics, the hermicity of the quantum
Hamiltonian). Not all photons that emitted emitted by excited molecules
escape to infinity, some are reabsorbed by the gas.

But you do have the right idea. A hot gas will radiate and loose energy
this way. This is simply a statement of the Stefan-Boltzmann law. A
black body radiates at a power proportional to its surface area and the
fourth power of its temperature[1]. So, eventually it will loose energy
until its temperature equilibrates with its surroundings, for example
to room temperature in a lab on Earth, or to 2.7K in a vacuum in outer
space (that's the temperature of the Cosmic Microwave Background
radiation). But it may take a *very* long time to radiate away that much
energy, especially since the cooler a gas is, the slower it will lose
energy.

That's what happens if the amount of gas is small. If you are
interested in a cloud of gas that has, say, a few solar masses in it.
Then it will also radiate and loose energy. However, as it cools,
gravity starts becoming more and more important and the cloud begins to
collapse onto itself under the attraction of its own gravitational
field. The collapse increases the pressure, which in turn increases the
temperature, until thermal motions balance out gravity once again. If
the pressure is high enough, it can even ignite things like nuclear
fusion. That's where interesting objects like stars make an appearance.
It is well known that stars loose energy through radation. But because
they have an internal energy source, it takes a long time (perhaps
billions of years) to loose enough energy for something interesting to
happen (supernova).

vivishek <vivishek.sudhir@gmail.com> wrote in news:1152006559.767761.231660
@j8g2000cwa.googlegroups.com:

Quote:

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

this doesn't happen in real life because the energy acquired by
colliding gas molecules is insufficient to drive the electrons to
higher energy states.

At room temperature it would be much more common with molecules getting
vibrational or rotational states excited whereupon they would emit an IR-
photon, but the principle is the same. Igor has the correct answer when
stating that the process is reversible. Assuming the surroundings are as
warm as the gas it will absorb photons as well and convert this energy back
to kinetic energy. All bodies emit and absorb photons this way, and if they
are at the same temperature as the surroundings these will match.

Quote:

Moreover how is that the electrons falling back
to lower states drive the gas to a solid state?

If energy was just lost by being emitted as photons the gas would cool down
until it condensed.

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?

[Good answer, I hardly need say, snipped]

I just wanted to add that in an open universe, such as current
observations and theory indicate, once nuclear interactions burn out, it
is in fact the fate of all matter eventually to stop glowing and grow
ever colder. Even the cosmic microwave background will continue to grow
colder as the universe continues to expand. But it will take many 10's
or even 100's of billions of years, so you don't need to worry just yet
:-)

I just read that an electron can move from an inner orbit to an outer
orbit by absorbing kinetic energy through collisions. So why don't
gases just glow themselves down into a solid state, with collisions
pushing electrons up an energy level, and photons being emitted as they
fall back down an energy level?